Low power electric immersion heaters are common with home spas.
2.
Heating up water for a bath using a coal fired boiler or electric immersion heater was a long and comparatively expensive business and you only got one shot at getting the temperature right.
3.
Where hot-water space heating boilers are installed, DHW cylinders are usually heated indirectly by primary water from the boiler, or by an electric immersion heater ( often as backup to the boiler ).
4.
I assume you mean an electric immersion heater and you are in the USA . If not, there is more you need to tell us . talk ) 21 : 21, 6 July 2012 ( UTC)